III. Fuel Velocity = 10 m/s

In this first case fuel velocity is only 10 m/s.
Although fields shown below are instantaneous fields ( t= 44ms) they are similar to the averaged fields

III.1. Fuel Mass fraction

Fuel Mass Fraction after 100000 iterations (t = 44 ms)

III.2 Velocity Vectors

Velocity Vectors after 100000 iterations (t = 44 ms)

Firstly, we can note a flow  symmetry.
Two vortices  of fuel are observed along the wall after the fuel inlet. They are due to the different x_velocities of the two fluids.
The mixture takes place at the layer between the air and the fuel and comes from the friction forces between the vortex and the laminar flow.

Even the fuel velocity is not enough high to obtain homogeneous mixture at the outlet,  there is no numerical problem and simulation seems to be correct.